An alignment layer for aligning a liquid crystal is important for keeping the order of alignment of the liquid crystals and realizing optical characteristics based on refractive index anisotropy of liquid crystal molecules, and is an essential compositional member that constitutes a liquid crystal display element. Alignment of the liquid crystals significantly affects display characteristics of liquid crystal display elements, and thus various methods for aligning the liquid crystal have been investigated. The liquid crystal display elements can be broadly classified into two types, that is, a vertical alignment type and a horizontal alignment type.
A liquid crystal display element (sometimes referred to as a VA mode liquid crystal display element) using a liquid crystal layer of a vertical alignment type has been widely used in displays for their excellent display characteristics such as high contrast. However, since it cannot be said that the liquid crystal display element using a liquid crystal layer of a vertical alignment type necessarily has sufficient viewing angle characteristics, various methods have been investigated to improve the viewing angle characteristics. As a method for improving the viewing angle characteristics, a multi-domain vertical alignment mode (MVA mode) has become prevalent, in which a plurality of liquid crystal domains having different alignment directions is formed in one pixel (an alignment division structure is incorporated therein). In the MVA mode, it is necessary to control the tilt alignment of the liquid crystal molecules in order to form the alignment division structure, and as such a method, a method in which a slit (opening) or a rib (projection structure) is provided in electrodes, is used. However, with the use of the slit or the rib, the slit or the rib is linear unlike a case where a pretilt direction is defined by an alignment film used in a TN mode used in the related art, and thus, the ability to control the alignment for the liquid crystal molecules becomes uneven within a pixel, whereby a problem of generation of a distribution in the response speeds arises.
In addition, there is another problem that a region provided with a slit or a rib exhibits decreased optical transmittance, resulting in a decrease in display luminance.
As another method for controlling the tilt alignment, there is disclosed a polymer alignment support (PSA; Polymer Sustained Alignment) technology in which photo- or thermo-polymerizable monomers are incorporated into a liquid crystal, the monomers being polymerized while tilting the liquid crystal molecules by voltage application so that the tilt direction of the liquid crystal molecules is memorized (see PTL 1). This method can overcome the problem in the distribution of the response speeds or a decrease in the optical transmittance in the slit-and-rib method. However, this method faces a problem such as changes in characteristics caused by the addition of monomers in the liquid crystal material, difficulty in controlling the process, and adverse effects of the residual monomers.
In order to avoid these problems, it is preferable even for the VA mode liquid crystal display element to form an alignment division structure by controlling the tilt alignment with an alignment film. As a method of providing an ability to control the tilt alignment on the vertical alignment film, there is a rubbing method, in which an alignment film made of a polyimide or the like is applied onto a substrate, and then the alignment film is rubbed with rubbing cloth to control the alignment direction and the pretilt angle. However, it is difficult to form a precise alignment division structure by the rubbing method, and thus problems of static electricity caused by friction and generation of impurities arise.
Meanwhile, as one of liquid crystal display elements using a liquid crystal layer of the horizontal alignment type, there is an IPS mode liquid crystal display element. The IPS mode liquid crystal display element has little dependency on viewing angles, for example in contrast and color tone, and is widely used in displays due to its excellent display characteristics. In the IPS mode, in order to reduce viewing angle dependency in the black display and the color reproducibility, it is required to have a low pretilt angle of one degree or less on the electrode surface. Even when achieving the horizontal alignment, a rubbing method as a general alignment method is used. However, when a horizontal alignment treatment is carried out by a rubbing treatment of a polyimide alignment film, the pretilt angle provided to the liquid crystal molecules exceeds one degree, and thus, a problem that the display characteristics are deteriorated arises.
From these problems, in any alignment mode of the vertical alignment type and the horizontal alignment type, it is important to control the alignment direction and the pretilt angle using the alignment film so as to improve the display characteristics. As a method for controlling the tilt alignment with an alignment film, a photo-alignment method is known, in addition to the methods using rubbing treatment (see PTL 2). In the photo-alignment method, a precise alignment division structure can be formed easily by changing the illumination pattern of light, and generation of static electricity or impurities is difficult, as compared with the rubbing treatment since a non-contact treatment on the alignment film can be carried out, and thus, it is expected to solve the above-described problems and to improve the display characteristics.
As the materials which can be a photo-alignment layer for the liquid crystal display element, a compound having a photochemically isomerizable site, such as an azobenzene derivative (see PTL 3), a compound having a photochemically crosslinkable site, such as a cinnamic acid derivative, a coumarin derivative, and a chalcone derivative (see PTLs 4, 5, and 6), a compound causing an anisotropic photo-degradation, such as a polyimide derivative, and the like are known.
However, the photo-alignment method using these compounds has a problem such as a low voltage holding ratio (VHR), as compared with a case using an ordinary alignment film. Therefore, various characteristics such as reliability, which allows realization of performance for controlling the tilt alignment of the liquid crystals and use in active matrix driving, are required, and photo-alignment layers for liquid crystals, which satisfy the requirements, have been required. Incidentally, the voltage holding ratio (VHR) means how much voltage applied to each pixel in a liquid crystal display element is held for a predetermined period of time (for example, one frame in the liquid crystal display element, 16.7 msec).
As described above, there has been a demand for a liquid crystal alignment layer having a superior ability to control the alignment of the liquid crystals and the pretilt angles and further, a high voltage holding ratio (VHR).